The objective is to study the relationships of dietary fat level, source, and their interactions to endocrine function and 7,12-dimethylbenzanthracene (DMBA) breast carcinogenesis in rats. Diets varying in fat level (10% or 40% of total calories) and source (corn oil or beef tallow) will be used in all studies. Minerals, vitamins, and fiber will be supplied at a constant ratio per kilocalorie of dietary energy. Feed intake will be recorded. The effects of dietary fat on mammary carcinogenesis in intact, ovariectomized and ovariectomized-estrogen-implanted rats will be examined. We will determine if dietary fat modifies mammary carcinogenesis, circulating prolactin and pituitary prolactin concentrations in three strains of rats with varying sensitivities to DMBA. We will further characterize the effects of dietary fat on 1) the capacity of the pituitary to secrete prolactin, 2) the secretory and metabolic clearance rates of prolactin, and 3) the circulating forms of prolactin and their relative immunoactivities.